The present invention relates to an optical pickup device and to an objective optical element used for the optical pickup device, and in particular, to an optical pickup device complying with standards of a plurality of optical information recording media (optical discs) and to an objective optical element used for the optical pickup device.
Up to the present, there have been developed and manufactured optical pickup devices (which are also called optical heads or optical head devices) for conducting reproducing and recording of information for optical information recording media (which are also called optical discs or media) such as CD (compact disc) and DVD (digital video disc or digital versatile disc), and they have come into wide use generally.
In recent years, there have been made studies and developments for standards of optical information recording media which make high density information recording possible.
The optical pickup device of this kind converges a light flux emitted from a light source (a laser diode is mainly used) on an information recording surface of an optical disc through an optical system (composed of optical elements such as a beam regulating prism, a collimator, a beam splitter and an objective optical element) to form a spot, and then, converges reflected light from an information-recorded hole(which is also called a pit) on a sensor through the optical system again to reproduce information by converting into electric signals. In this case, a light flux of the reflected light varies depending on a form of the information-recorded hole, and therefore, by using this, information of “0” is distinguished from that of “1”. Incidentally, on an information recording surface of the optical disc, there is provided a protective substrate (a protective layer made of plastic which is also called a cover glass).
Further, when recording information on a recording type medium such as CD-R or CD-RW, a spot by means of a laser light source is formed on a recording surface to generate thermochemical changes in a recording material on the recording surface. Due to this, in the case of CD-R, irreversible changes are caused on thermo-diffusive dyes, and a shape identical to the information-recorded hole is formed. In the case of CD-RW wherein a phase change type material is used, reversible changes are made by thermochemical changes between the state of crystal and the amorphous state, and thereby, it is possible to rewrite information.
In the optical pickup device for reproducing information from an optical disc that is in compliance with CD standards, NA of an objective lens is about 0.45 and a wavelength of a working light source is about 785 nm. For recording, on the other hand, those with about 0.50 are usually used. Incidentally, a thickness of a protective substrate of an optical disc complying with CD standards is 1.2 mm.
As an optical information recording medium, CD has been popularized widely, and in recent years, there has been diffused DVD. The DVD is one wherein a capacity for information storage has been increased by making a protective substrate to be thinner and by making an information-recorded hole to be smaller, than those of CD, and a recording capacity of DVD is as large as about 4.7 GB (gigabyte) for that of CD which is about 600-700 MB (megabyte), and DVD is used in many cases as a distributed medium wherein an animation image such as a movie is recorded.
In the optical pickup device for reproducing information from an optical disc that is in compliance with DVD standards, NA of an objective lens is about 0.60 and a wavelength of a working light source is about 655 nm, because the information-recorded hole is small as stated above, although the principle of DVD is the same as that of CD. For recording, on the other hand, those with about 0.65 are usually used. Incidentally, a thickness of a protective substrate of an optical disc complying with DVD standards is 0.6 mm.
With respect to the optical disc in compliance with DVD standards, those of a recording type have already been put to practical use, and there are respective standards including DVD-RAM, DVD−RW/R and DVD+RW/R. Technical principles for these are the same as those in the case of CD standards.
An optical disc of a higher density and higher capacity type is now being proposed as stated above.
This is one employing mainly the so-called violet laser light source having a wavelength of about 405 nm.
With respect to the “high density optical disc” of this kind, a thickness of a protective substrate, a storage capacity and NA are not determined uniformly, even when a wording wavelength is determined.
If the direction for improving recording density substantially, a thickness of a protective substrate of the optical disc is required to be thin and NA is required to be great accordingly. On the contrary, it is also possible to make the thickness of a protective substrate and NA to be the same the standards of the conventional optical disc such as DVD. In that case, properties required as an optical system are relatively generous, though physical recording density is not improved substantially.
Specifically, with respect to a thickness of the protective substrate, a thickness that is further thinned to be as thin as 0.1 mm and a thickness of 0.6 mm that is identical to that of DVD have been proposed.
Though the plural standards for the “high density optical disc” of this kind are the same in principle as those for CD and DVD, if a thickness of the protective substrate is different, a size of the information-recorded hole is also different. Therefore, even if a light source having the same wavelength is used, it is not possible to conduct reproducing and recording of information by using the same optical pickup device simply.
Therefore, some problems need to be solved for attaining “compatibility” which makes it possible to conduct reproducing and recording of information for both “high density optical disc” and conventional plural standards, with a single optical pickup device including an objective optical element.    (1) Since an appropriate NA varies depending on each optical disc, “diaphragm” functions to use respective NA selectively for media are required.    (2) On optical discs each having a different thickness of a protective substrate, spherical aberrations are caused by the thickness difference, which needs to be solved.
In particular, extremely remarkable spherical aberration is caused for the difference between a thickness of 0.6 mm for DVD and that of 1.2 mm for CD. Further, when a thickness of the protective substrate of “high density optical disc” is made to be 0.1 mm, further correction of spherical aberration is needed.
For these problems, there have been proposed a method to use a dichroic filter having a property to select wavelengths and a method to give a property to select wavelengths by providing a diffractive structure or an optical path difference furnishing structure on an objective optical element, and they have been realized.
There also exist the following problems.    (3) When attaining “compatibility” between standards of optical discs wherein a thickness of a protective substrate and NA are the same (including mostly the same) for both optical discs and a working wavelength is different (for example, when using 655 nm and 405 nm), occurrence of spherical aberration (=spherical aberration equivalent to an amount of chromatic aberration) caused by the wavelength difference is unavoidable, although spherical aberration caused by a difference of protective substrate thickness does not occur, which needs to be solved.    (4) With respect to light fluxes to enter an objective optical element, when all light fluxes are collimated to be infinite parallel light even when a wavelength is different, spherical aberration based on a magnification difference is not caused, but in the case of finite light (divergent light and convergent light), a difference of magnification is caused, and spherical aberration based on this difference of magnification is caused.    (5) When attaining “compatibility” for three formats of CD, DVD and “high density optical disc” by using a single optical pickup device, it is necessary to conduct spherical aberration correction which is more complicated than the occasion for “compatibility” between two formats. In other words, corrections need to be made ‘between CD and DVD’, ‘between CD and “high density optical disk”’and ‘between DVD and “high density optical disk”’, for the correction only between CD and DVD in the case of two formats.
In Patent Document 1, there is disclosed a compatible optical pickup device that is compatible between the “high density optical disc” and DVD both mentioned in the present invention, which has an optical element having a diffractive structure in its optical path, and forms a converged-light spots by diffracted light with different orders such as second order diffraction for “high density optical disk” and first order diffraction for DVD or third order diffraction for “high density optical disk” and second order diffraction for DVD.
However, there is neither disclosure nor suggestion for technologies coping with three types of formats such as those in the present invention.
Patent Document 1
TOKKAI No. 2001-93179
As stated above, for “compatibility” between different optical discs, there have been proposed a method to use a dichroic filter having a property to select wavelengths and a method to give a property to select wavelengths by providing a diffractive structure on an objective optical element, and they have been realized.
However, there is a fear that optical performances are not attained when an objective optical element is made to have various optical functions, although it is preferable for reduction of the number of parts, low cost and downsizing.
When attaining the compatibility for three formats which is to be solved this time, it is impossible to solve by using simply the aforesaid methods, because of many objects to be corrected.
Further, spherical aberration is sometimes caused also by the magnification of a light flux entering an objective optical element, which needs to be solved.
Since the optical pickup device itself is required to be small in size, light in weight and thin in thickness, factor parts, in particular, optical elements are required to have strict capabilities.
When the optical pickup device is made thin, in particular, a working distance (distance from an objective optical element to an optical disc) becomes short. Even if the working distance is made to be great by enhancing the magnification, image height characteristics are worsened, which is not preferable. If a difference of working distances is great, a load on an actuator is increased, and power consumption is also increased.
In Patent Document 2, there is disclosed an optical disc device that is approximately composed of the first-third light sources which emit light respectively of the first-third wavelengths, an objective lens that receives light of the firs-third wavelengths respectively to converge them on a prescribed optical information recording medium and of a collimator lens.
Light having respectively the first and the second wavelengths emitted respectively from the first and the second light sources pass through the collimator lens, and in this case, light with the first wavelength is collimated by the collimator lens to be parallel light and enters the objective lens, while, light with the second wavelength is not collimated and enters the objective lens as divergent light. Further, light with the third wavelength emitted from the third light source does not pass through the collimator lens and enters the objective lens directly as a divergent light.
Light each having one of the first-third wavelengths emerging from the objective lens is converged on each of three types of optical information recording media such as a high density optical disk, DVD and CD, each having a different wavelength and a different protective substrate thickness, thus, recording and/or reproducing for various types of information is conducted.
Patent Document 2
TOKKAI No. 2001-43559
Incidentally, in the apparatus disclosed in Patent Document 2, light with the first wavelength enters an objective lens as a parallel light as stated above, while, light with the second wavelength and light with the third wavelength enter the objective lens as divergent light, and thus, an optical system magnification of a light-converging optical system is different from others for three types of optical information recording media.
Therefore, for example, an optical path for light with each of the first-third wavelengths is different from others, and therefore, a plurality of optical elements are required to be arranged so that each of them may correspond to each optical path, and thereby, there have been problems that a structure of an optical disc device is complicated and the number of parts of the apparatus is increased.
Further, there have been problems that image height characteristics are worsened and various aberrations such as coma and astigmatism are caused, in the case of tracking to move an objective lens for an optical disc when conducting reproducing and recording for the optical disc, because divergent light enters the objective lens.
There has further been a problem that spherical aberration caused by temperature changes is greater, compared with the so-called apparatus of an infinity system type wherein parallel light enters an objective lens.